A discussion with a friend about "back priming" interior custom trim has me wondering. He has said that priming the back side of the trim before it goes up, especially with gas heat, will keep the wood from expanding and avoiding cracks in the caulking. We were mainly talking about multi-piece crown. The idea was that it keeps the moisture or humidity from working through the wood from the air pockets behind it.

He's been painting for 40 years, and I won't pretend that I know more, but I would think the latex primers and finishes of today will allow that moisture to come through. Anyone do this now, or have done it, or know what I am talking about? If so can you enlighten me a bit?

A discussion with a friend about "back priming" interior custom trim has me wondering. He has said that priming the back side of the trim before it goes up, especially with gas heat, will keep the wood from expanding and avoiding cracks in the caulking. We were mainly talking about multi-piece crown. The idea was that it keeps the moisture or humidity from working through the wood from the air pockets behind it.

He's been painting for 40 years, and I won't pretend that I know more, but I would think the latex primers and finishes of today will allow that moisture to come through. Anyone do this now, or have done it, or know what I am talking about? If so can you enlighten me a bit?

We do alot of large scale high end interior prefinishing. It is very rare that we backprime. Wood expands and contracts, period.

Some example of backpriming would be baseboard and shoe (floors often get wet mopped), wainscot in bathrooms or excessive moisture areas, etc. Sometimes if there is an interior frieze detail that is unusually wide (10-12") we will do it as a measure to help prevent cupping. For most standard size interior casing and trim elements, no. Exterior? absotutely

I would love to know more about that...we should have our own mythbusters,
we could call it "Paintbusters" maybe Slick and Pro could host...make a vidio and everything....

I got deep into the questions of expansion and contraction a while back when a jobs trim gave way a year later...I would still like to learn more, but finding info on it isn't easy....and I would think it might be some kind of chemistry discussion that I couldn't follow. Any more info would be greatly appreciated. Thanx

....priming the back side of the trim before it goes up, especially with gas heat, will keep the wood from expanding and avoiding cracks in the caulking....The idea was that it keeps the moisture or humidity from working through the wood from the air pockets behind it.

Sounds like a crock to me

Sometimes we learn something from our mentor, maybe he/she got it from theres
We're not sure why it started originally, but it works
So we keep doing it
Maybe when we are older, years later at the bar with our painting buddies we wonder...or maybe even we are teaching the new kid, she asks...why?

Uh.......uh......I think it keeps the humidity from cracking the caulk?

Well, if you were the new kid, and you were discussing this now 30 years later, it makes perfect sense because you've been doing it for 30 years

But really it was just a crock
Oldy Olsen started doing it 50 years ago because it seemed like the right thing to do to him, or was getting paid by the hour, or maybe back in '54 with the paints and caulk back then it did help...maybe it was helpful with vintage '54 caulk and linseed oil paints

But now it's not needed

But I digress....it just "sounds" like a crock to me...I can't see it

We need PaintBusters

__________________"History shows again and again how nature puts up with the folly of men"
-Blue Oyster Cult

Its not totally an old wise tale. Its based on sound finishing theory, I think. The reason its overkill for interior is that most houses today have much more sophisticated air quality systems than the did 50-100 years ago, when interior air was pretty much the same as exterior air.

Speaking just for us, we prefinish raw materials that come into our hands at 6-8% m.c. They end up in houses with systems so insane that we can go back a year later (after 4 very different Vermont seasons) and probe with a moisture meter and not get a reading over 10. But as I said, exterior...whole different story.

I am assuming that everyone here understands the basic premise of wood movement. Very basically summarized by expansion in the summer, contraction in the winter. Kind of like paint companies...

Its not totally an old wise tale. Its based on sound finishing theory, I think.

Yessir! I once worked for a GC that had us do this to any wood we replaced, interior or exterior. He also required us to paint all 6 sides of any new door.

My Grandfather who taught me the w/c trade used to backprime everything too. Maybe the technique got lost over the years due to better quality finishes. Or, more likely, got set aside as more hacks and blow-n-gos came along and needed to scrimp on a step to make any jack.

Right on Pro. On doors its still important, whether interior or exterior, because a door has lots of pieces of wood expanding and contracting all at once and joinery and endgrains...its such a simple habit and another case of an ounce of protection beating a pound of cure. And you are right, its kind of neat to be aware of the tradition. Whenever I get a chance to talk to an old time carpenter, I am all ears...

Some example of backpriming would be baseboard and shoe (floors often get wet mopped), wainscot in bathrooms or excessive moisture areas, etc. Sometimes if there is an interior frieze detail that is unusually wide (10-12") we will do it as a measure to help prevent cupping. For most standard size interior casing and trim elements, no. Exterior? absotutely

Thanks Vermont,

Any wood that even might be in a high moisture area I agree that it should be protected even if it is overkill.

As far as the back priming normal trim he offered proof - on a high end condo the GC started installing the trim before they arrived on site to back prime it. The trim man already had half of it installed but they were able to pre-prime the other HALF of the wood that was on the site before it went up. A few months later while on a different trip to the house after completion the GC was irritated and wanted to know if they caulked it properly or what they did wrong because the caulking on the first half of the job was failing. My friend took the GC to the other half that was back primed and it had no failing or flaws and he reminded the GC that the trim was put up when it wasn't primed therefore the cost went to the GC.

I Dunno!

I do know that some paint manufacturers have said that using a waterborne primer was better than an alkyd because it allowed the wood to breath and was more flexible where the Alkyd did not and was not.

I Dunno!

Personally I think its a waste of time but I would like a few more opinions before I respectfully disagree with him and get pounded down

Good points. Again, I say alot of the success of the trim depends on the interior humidity conditions, season, etc. If it was summer and really humid and of course the house is wide open during construction, then certainly the backprimed stuff would be better suited for survival. The caulking failure is likely a result of non-backprimed boards cupping and twisting and splitting the caulk?

When a client is persistent in having crown molding installed in a bath (we do not suggest it unless the bath has high ceilings) we do back prime. We usually install pre-primed MDF crown. In other drier rooms we do not back prime. I have heard that back priming all trims helps prevent joints separating at joints.

I have heard that back priming all trims helps prevent joints separating at joints.
Jerry

Again, it certainly couldnt hurt. The argument against it is that wood (unlike pvc and other synthetics) expands and contracts in width not in length. So you are more likely to see cracked caulking than joint separation. Common miter joints wouldnt see the benefit of it. We see alot of multi-piece trim schemes with butt-jointed square stock flanked by separate inner beads and backbands. In this situation there is a butt joint in the square stock that could benefit from backpriming. But the carpenters usually glue and biscuit those joints, so the wood will break on the grain before the joint will separate. And, as I said earlier, the houses we work in are so carefully controlled for humidity that its rarely a problem. If I was to make a crusade for backsealing anything, it would be wood floors. You see a lot more floors buckling and cupping than anything because of the sheer number of pieces all expanding and contracting together in a floor system, but thats another topic. I would also like to begin offering finishes on framing lumber, but I guess I am a nut that way.

I think more than anything that causes this contraction expansion - the $hitty soft pine that is cut down these days - old growth wood is so dense - it just can't accept the moisture that lumbered woods of today do. Have you ever mitered cut a piece of trim these days? You can practically see each individual wood cells - they're so big.

Arguments about back priming and doing all six sides of a door have gone on for years now. In many cases especially exterior I believe it is helpful, even though cracked panels on real wood doors have little to do with the top or bottom being primed. We did 8 new condos last year with real wood fir doors, stained and finished with 3 coats of helmsman spar. 1 of the doors had a veneer issue on the lower stile. Factory rep showed up with a mirror and the first thing he did was check the top and bottoms of all the doors. Luckily they were all done and he admitted that the problem was movement under the veneer with the stave's used to assemble the door but if the tops were not primed they would not guarantee the door. He told me they get out of many problems because most painters do not edge prime. These condos have beautiful pre finished oak flooring in various colors and widths. I asked him if it was so important then why don't they apply at least 1 coat on the back side of pre finished flooring. All he could say was "good question" They expand and contract like all woods do and when it's excessive they give you that excuse. No mirrors used on their end.

It makes sense to back prime in certain situations concerning moisture ect...

It can also set a business apart from another for quality.

Having read through this post it would seem to have a benefit. What could it hurt to use this as a default in the processes of our work? A bit more time and money, but heck, anything to keep from returning due to rework is a greater investment and makes the business that much better in the eye of the clients. Why risk the chance of anything happening? If back-priming can eliminate a risk..... I'd say "Just do it!".

Quality is not just the finished product, it's mostly in the processes to the finished product.